Overview
java.io.FileClasses are abstract representations of file and directory pathnames, and are mainly used for operations such as file and directory creation, search, and deletion.
Construction method
public File(String pathname): Creates a new File instance by converting the given pathname string to an abstract pathname.public File(String parent, String child): Creates a new File instance from the parent pathname string and child pathname string .public File(File parent, String child): Creates a new File instance from the parent abstract pathname and child pathname strings .- The construction example, the code is as follows:
// File path name
String pathname = "D:\\aaa.txt";
File file1 = new File(pathname);
// File path name
String pathname2 = "D:\\aaa\\bbb.txt";
File file2 = new File(pathname2);
// By parent path and child path string
String parent = "d:\\aaa";
String child = "bbb.txt";
File file3 = new File(parent, child);
// By parent File object and child path string
File parentDir = new File("d:\\aaa");
String child = "bbb.txt";
File file4 = new File(parentDir, child);
💡Tips:
- A File object represents a file or directory that actually exists on the hard disk.
- Regardless of whether there is a file or directory in the path, it does not affect the creation of the File object.
Common method
How to get features
public String getAbsolutePath(): Returns the absolute pathname string of this File.public String getPath(): Convert this File to a pathname string.public String getName(): Returns the name of the file or directory represented by this File.public long length(): Returns the length of the file represented by this File.
Method demonstration, the code is as follows:
public class FileGet {
public static void main(String[] args) {
File f = new File("d:/aaa/bbb.java");
System.out.println("Absolute path to the file:"+f.getAbsolutePath());
System.out.println("File construction path:"+f.getPath());
System.out.println("File name: "+f.getName());
System.out.println("File length: "+f.length()+"bytes");
File f2 = new File("d:/aaa");
System.out.println("Absolute path to directory:"+f2.getAbsolutePath());
System.out.println("Directory construction path:"+f2.getPath());
System.out.println("Catalog Name:"+f2.getName());
System.out.println("Directory length:"+f2.length());
}
}
Output results.
File absolute path :d:\aaa\bbb. java
File construction path :d:\aaa\bbb. java
File name :bbb. java
File length :636 bytes
Directory absolute path :d:\aaa
Directory construction path :d:\aaa
Directory name :aaa
Directory length:4096
Description in the API: length(), which indicates the length of the file. But the File object represents a directory, the return value is unspecified.
Absolute and relative paths
- Absolute path : The path starting from the drive letter, this is a complete path.
- Relative path : The path relative to the project directory, this is a convenient path and is often used in development.
public class FilePath {
public static void main(String[] args) {
//The bbb.java file under the D drive
File f = new File("D:\\bbb.java");
System.out.println(f.getAbsolutePath());
// The bbb.java file under the project
File f2 = new File("bbb.java");
System.out.println(f2.getAbsolutePath());
}
}
Output results.
D:\bbb.java
D:\idea_project_test4\bbb.java
How to judge function
public boolean exists(): Whether the file or directory represented by this File actually exists.public boolean isDirectory(): Whether this File represents a directory.public boolean isFile(): Whether this File represents a file or not.
Method demonstration, the code is as follows:
public class FileIs {
public static void main(String[] args) {
File f = new File("d:\\aaa\\bbb.java");
File f2 = new File("d:\\aaa");
System.out.println("d:\\aaa\\bbb.java Does it exist:"+f.exists());
System.out.println("d:\\aaa Does it exist:"+f2.exists());
// Determine if it is a file or a directory
System.out.println("d:\\aaa file?:"+f2.isFile());
System.out.println("d:\\aaa Catalog?:"+f2.isDirectory());
}
}
Outcome:
d:\aaa\bbb.java Does it exist:true
d:\aaa Does it exist:true
d:\aaa file?:false
d:\aaa Catalog?:true
How to create a delete function
public boolean createNewFile(): Creates a new empty file if and only if a file with that name does not already exist.public boolean delete(): Deletes the file or directory represented by this File.public boolean mkdir(): Creates the directory represented by this File.public boolean mkdirs(): Creates the directory represented by this File, including any required but non-existing parent directories.
Method demonstration, the code is as follows:
public class FileCreateDelete {
public static void main(String[] args) throws IOException {
File f = new File("aaa.txt");
System.out.println("Does it exist:"+f.exists()); // false
System.out.println("Whether to create:"+f.createNewFile()); // true
System.out.println("Does it exist:"+f.exists()); // true
File f2= new File("newDir");
System.out.println("Does it exist:"+f2.exists());// false
System.out.println("Whether to create:"+f2.mkdir()); // true
System.out.println("Does it exist:"+f2.exists());// true
File f3= new File("newDira\\newDirb");
System.out.println(f3.mkdir());// false
File f4= new File("newDira\\newDirb");
System.out.println(f4.mkdirs());// true
System.out.println(f.delete());// true
System.out.println(f2.delete());// true
System.out.println(f4.delete());// false
}
}
💡Description in API: delete method, if this File represents a directory, the directory must be empty to delete.
directory traversal
public String[] list(): Returns a String array representing all subfiles or directories in the File directory.public File[] listFiles(): Returns a File array, representing all subfiles or directories in the File directory.
public class FileFor {
public static void main(String[] args) {
File dir = new File("d:\\java_code");
//Get the names of the files and folders in the current directory.
String[] names = dir.list();
for(String name : names){
System.out.println(name);
}
//Get the file and folder objects in the current directory, as long as you get the file object, then you can get more information
File[] files = dir.listFiles();
for (File file : files) {
System.out.println(file);
}
}
}
💡Tips:
The File object that calls the listFiles method must represent the actual directory, otherwise it returns null and cannot be traversed.
